Improve AbstractAnalysisData data access interface.

Improve AbstractAnalysisData data access interface.

Access to stored data is now done through wrapper object instead of
retrieving raw pointers.  Also refactored the implementation such that
the pure virtual methods that implement the actual data access are now
protected, and the public members are now simple non-virtual wrappers
that perform common checking and indexing.

Also removed support for negative data frame indexing.  It is no longer
necessary because the data frame index is now passed to
AnalysisDataModuleInterface::pointsAdded(), so it's easy to compute the
absolute index of past frames, and additionally it didn't work properly
in parallelization schemes.  Also removed possibility to access the
current frame from pointsAdded(), as it would just create complications
to allow access to an incomplete frame.

Part of issue #827.

Change-Id: If932c3f6b8394f8ceacc632fb9d87ec1933f56ca